1. Field of the Invention
The present invention relates to an LCD module used as a display device of a data processing apparatus such as a computer, and more specifically, to a back light type LCD module for lighting a display portion by using a light source.
2. Description of the Related Art
Recently, back light type LCD (Liquid Crystal Display) modules are used in many laptop personal computers and wordprocessors. In a back light type LCD module, a difference in lightness between a background of a display surface and an LCD surface is large so that the operator can easily recognize characters and figures on the LCD surface. In general, in an LCD module, light is generated by cold-cathode tubes provided as light sources on side surfaces of the LCD panel. The generated light enters into in a light-guide panel provided at a position opposite to the LCD surface as the display surface of the LCD panel. Light entered in the light-guide plate is uniformly radiated on the entire LCD panel.
In such an LCD module, heat is also generated by the cold-cathode tubes together with light, and display non-uniformity occurs on the LCD surface due to the heat. Display non-uniformity occurs when a threshold voltage for changing the orientation of the liquid crystal of the LCD is changed in accordance with a change in temperature of the liquid crystal. For example, the threshold voltage is low at a high temperature liquid crystal region and is high at a low temperature liquid crystal region.
FIG. 1 shows the structure of the rear portion of the LCD module and its temperature distribution. The temperature decreases in directions indicated by arrows. In an LCD module having a light-guide plate 12 and an LCD panel 15, the closer to cold-cathode tubes 11a and 11b to which the voltage is supplied from a power supply (not shown) via a power line 19, the higher the temperature of the LCD surface because of the heat generated by the cold-cathode tubes 11a and 11b. A portion far from the cold-cathode tubes 11a and 11b has a low temperature as it is not much influenced by the heat generated by the cold-cathode tubes 11a and 11b.
Normally, the LCD module is used at a predetermined angle to face its display surface toward a face of an operator. For example, when the LCD module is placed to face the display surface toward an operator (note that sides 20 and 21 correspond to upper and lower portions, respectively, of the LCD module), the cold-cathode tubes 11a and 11b are located at the lower and upper portions, respectively. Thus, heat generated by the cold-cathode tubes 11a and 11b moves upward, as shown in FIG. 1, because of air convection. Accordingly, high temperature regions are formed near the cold-cathode tubes 11a and 11b, and a low temperature region is formed at a portion higher than the central portion of the LCD panel.
Conventionally, the cold-cathode tubes are arranged at portions farthest from the LCD panel in order to minimize influence of heat generated by the cold-cathode tubes, thereby preventing display non-uniformity. In this case, however, since the whole size of the LCD module is increased, this is not applicable to a small-size data processing apparatus. Further, in order to minimize influence of heat and to prevent display non-uniformity, openings are arranged in a panel cover, thereby dissipating heat generated by the cold-cathode tubes to the outside. However, since convection of air does not occur, degree of display non-uniformity of an LCD panel increases.
From the above situation, an LCD module is demanded in which display non-uniformity caused by heat generated by the cold-cathode tubes can be prevented while the positional relationship between the cold-cathode tubes and the LCD panel is set in the same manner as in the conventional LCD module.